The equal channel angular pressing (ECAP) of the AZ91 magnesium alloy was tested from 553 K to 693 K. The initial grain size of the investigated alloy in homogenized state was about 150 mu m. Samples were deformed through a die characterized by an inner contact angle. During the ECAP process the microstructure changed homogeneously and exhibited a decrease of grain size to 10 mu m. Transmission electron microscopy allowed the observation of a high dislocation density and large number of twins and shear bands in the deformed material. Some regions in the investigated alloys exhibited a dynamic recrystallisation process.